It's highly unusual to see someone for 6 months and not realize that they're still living with their spouse and kids. Even if the other person tells you it's not the case.
We don't know what kind of "seeing" they were doing though - "dating" someone in modern lingo can mean anything from casual sex twice weekly to having breakfast every morning together. There's a huge variance in the type of information you know about someone between those two types of relationships.